The materials described in this invention partially relate to constituents defined in a prior patents, U.S. Pat. No. 6,099,605 and its Divisional, U.S. Pat. No. 6,432,855; the first issued Aug. 8, 2000 and the second Aug. 13, 2002. Those patents describe a ceramic material which is based on an orthorhombic boride of the general formula: AlMgB14. The ceramic is super abrasive, and in most instances exhibits a hardness of 30 GPa or greater. This invention relates to new compositions involving the use of AlMgB14 and TiB2 together in high percent TiB2 formulations as a strengthening reinforcement to provide composites of extremely high erosive and abrasive wear resistance.
Erosive and abrasive wear may both be viewed as surface damage resulting from the relative motion with another body. Where the two forms of wear diverge involves the nature of the relative motion. According to ASTM G40, abrasive wear is degradation ‘due to hard particles or hard protuberances forced against and moving along a solid surface;’ whereas erosive wear is defined as ‘the progressive loss of original material from a solid surface due to mechanical interaction between that surface and a fluid, a multi-component fluid, or impinging liquid or solid particles.’ Erosion may be viewed as mechanical or physical surface damage resulting from impingement by solid particles or liquid droplets. Erosive wear is a function of the number of impacts and the momentum transferred per impact, and is typically measured by the change in mass (or volume) of a material after exposure to an abrasive particle flux. Variables include impingement angle, particle speed upon impact, particle morphology, and duration of erosion.
Ultra-hard materials are commonly used for wear-resistant applications and also for advanced cutting tools. Such materials are needed in many application such as earth moving, mining, abrasive slurry transport, rock drilling etc. where they experience sliding or impacting interaction with abrasive particles. Wear is also important in processes such as grinding, lapping and polishing that are used for shaping materials to conform to precise dimensions or to achieve smooth surface finish. Since super hard materials cannot be machined by a conventional single-point cutting process, abrasive machining is the only feasible process to finish these materials.
In view of its commercial significance, many researchers have tried to study the mechanisms involved in wear. Others have studied these mechanisms with the objective of increasing material removal efficiency in abrasive machining of hard materials. The problem with enhancing the material removal rates is the surface and subsurface damage that occurs which is detrimental to mechanical properties. In their studies on hard ceramic materials, some workers have concluded that the major surface damage patterns due to grinding are microplastic deformation, fracture chipping pits, microcracks and fragmentation in the damaged region of the material. At sufficiently low loads or with small particles, fracture may be suppressed and abrasive wear may occur by plastic deformation. At higher loads or with larger particles, brittle facture occurs leading to a much higher wear rate. According to Gee (M. G. Gee, Low load multiple scratch tests of ceramics and hard metals, Wear, Vol. 250, 2001, pp. 264-281), fracture of hard metals occurs on a fine scale and fracture of ceramics occurs on a large scale, often removing large fragments of material. Malkin and Ritter (S. Malkin, J. E. Ritter, Grinding mechanisms and strength degradation for ceramics, J. Eng. Ind., ASME Trans. 111 (1989) 167-174) studied the mechanisms of grinding of ceramics. They concluded that at low loads material removal occurred by plastic deformation and at higher loads by fracture. In the latter case, the finished surface was highly fragmented, and the strength after grinding was lower.
As for the effect of abrasive machining, the strength of ceramics decreases due to the increase in the subsurface damage caused by grinding and abrasive machining (T. J. Strakna, S. Jahanmir, R. I. Allor, K. V. Kumar, Influence of grinding direction on fracture strength of silicon nitride, J. Eng. Mater. Tech., ASME Trans. 118 (1996) 335-342). Though the surface after machining often appears smooth, cracks have been detected below the surface (H. H. K. Xu, L. Wei, S. Jahanmir, Grinding force and microcrack density in abrasive machining of silicon nitride, J. Mater. Res. 10 (12) (1995) 3204-3209). This damage has been attributed to the pileup of residual stresses from mechanical and thermal effects. Johnson-Walls and Evans (D. Johnson-Walls, A. G. Evans, Residual stresses in machined ceramic surfaces, J. Am. Ceram. Soc. 69 (1986) 44-47) studied the residual stresses in ceramics and concluded that the intensity of stresses increased with hardness and was also influenced by other material properties such as fracture toughness and modulus of elasticity.
It has been suggested that fracture toughness and hardness are the most important mechanical properties affecting the abrasion of brittle materials. Gahr (K. H. Z. Gahr, Microstructure and Wear of Materials, Elsevier, Amsterdam, 1987, pp. 180181) and Mao (Mao, D. S (Zhejiang Univ); Li, J.; Guo, S. Y., Study of abrasion behavior of an advanced Al203-TiC—Co ceramic, Wear, v 209, n 1-2, Aug, 1997, p 153-159) studied the abrasion wear resistance of several alumina ceramics and concluded that wear resistance was governed primarily by the toughness of the ceramic. The early work by Khrushchov and Babichev (Khrushchov M M, Babichev M A. Friction and Wear in Machinery. 1958;12:1-13) on pure metals showed that abrasion rates were inversely proportional to hardness. They also reported that abrasion was affected by several other material parameters such as elastic modulus, yield strength, crystal structure, microstructure, and composition.
Other factors affecting wear include the type of abrasive and its characteristics, speed and angle of contact, unit load of abrasive on the material, humidity, and temperature. In the case of most abrasives, hardness, toughness, angularity, and size are the important parameters (Nathan G K, Jones W J D. Proceed. Instn. of Mechanical Engineers. 1966-67;181:215-221; Avery H S. The Measurement of Wear Resistance. Case Report 340-10, Dept. Report9AE-134, American Brake Shoe Company, 1961). The shape of abrasive particles together with load influences the shape of the grooves produced in the material and transition from elastic-to-plastic contact. In belt abrasion, coolant is important for enhanced belt life, effective material removal, and reduced surface damage.
Advanced materials for wear-intensive applications must possess both toughness and hardness. Hardness is needed to resist localized plastic deformation and flow while toughness is needed to prevent microfracture and chipping. Cemented carbide (WC—Co), which is mostly WC with Co as binder, has found uses in numerous wear-intensive applications because of its moderately high hardness and high toughness. While c-BN (cubic boron nitride) is the hardest material next to diamond, its low fracture toughness makes this material prone to microfracture and chipping. Consequently, monolithic c-BN has limited utility in most wear-intensive applications. The same argument can be applied to diamond; its relatively low fracture toughness, combined with high cost, makes diamond unattractive for most wear applications.
A recent study of complex ternary borides revealed a new class of lightweight, highly wear-resistant ceramic composites. These composites of the form M1M2Bz plus M3B2, where M1 and M2 are metal-like elements, where Z>=12, where M3 is a Group IVB transition metal element (Zr, Ti, Hf), and where the weight percentage of M3B2 occurs in the range from 40 to 90 percent (28 to 85 volume percent). were prepared by mechanical alloying, a high energy solid state technique, and consolidated by hot pressing. The resultant composite possesses an ultra-fine microstructure, with a typical phase spacing on the order of 50 to 200 nm. Wear resistance is strongly affected by porosity, and maximum wear resistance is achieved when the density of the composite reaches 100%.
This invention constitutes an unexpected and unanticipated departure from compositions specified in our prior U.S. Pat. Nos. 6,099,605 and 6,432,855. In these prior inventions, a ceramic material which is an orthorhombic boride of the general formula: AlMgB14:X, with X being a doping agent or additive is made. The ceramic is a super abrasive, and in most instances provides a hardness of 40 GPa or greater, see the invention of U.S. Pat. No. 6,099,605, the disclosure of which is incorporated herein by reference.
A primary object of the present invention is to provide a new class of ceramic composites which consist of M1M2B14 and M3B2, where M1 and M2 are comprised of metal-like elements and M3 is taken from the group (Zr, Ti, Hf) or combinations thereof, but at unexpectedly high levels of M3B2 to achieve high wear resistance, which is the ability of the material to withstand impingement by abrasive particles. In particular, it has been discovered that the wear resistance of M1M2B14 where M1=Al and M2=Mg can be dramatically improved when the amount of M3B2 where M3=Ti is within the range of 40% to 90% by weight. (28.3 to 85 volume %). This is contrary to expectations from previous art teachings.